Haunted cupboard?

Today, my wife and I were out and about. My wife wanted to visit the owner of a high end antique furniture store that is located in the same town her store is  located. She wanted to discus some matters with the store owner. When we arrived the store was heavily patronized and so we entertained ourselves looking at the beautiful furniture that had come in since we were last there. But customers tend to be like bananas, they come in bunches. Finally a pause occured and my wife and the store lady had a chance to catch up. After a bit, the store owner said, "Can a piece of furniture be haunted?" This may sound strange or funny for someone to ask; but the lady knows us well enough to know we will take the question seriously. She continued by sayng her husband had purchased  this antique, large, old kitchen cupboard for resale in their store. They brought it in and displayed it in the middle of the main showroom floor. I inspected it. It is a very solidly built piece of furniture. It has two large glassed-in doors. The glass in the doors is very secure. So what about it being "haunted". She says that since that piece of furniture was brought in she has experienced a loud tapping on glass. Like some one tapping the glass with a coin or fingernail. At first she thought someone was tapping on her front windows trying to get her attention. But there was no one there. This occured several times; more often than not when she is in another room of her store. Like she says," it can be very disconcerting when you think you are the only one there". Because the event occurs several times a day, she has narrowed down the source of the noise to this large old cupboard. I have read stories about "haunted furniture" but I have never experienced such. I would rather think the cause was something more mundane, but I haven't a clue what it could be. Anyone out there know how a piece of furnituire could make tapping noises?